@NotNull, @NotBlank, @NotEmpty in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference between @NotNull and @NotEmpty

Which statement correctly describes the difference between @NotNull and @NotEmpty annotations in Spring Boot validation?

A <code>@NotNull</code> ensures the value is not null, but allows empty strings or collections; <code>@NotEmpty</code> ensures the value is not null and not empty (for strings, collections).

B <code>@NotNull</code> only works on strings; <code>@NotEmpty</code> only works on collections.

C <code>@NotNull</code> ensures the value is not null and not empty; <code>@NotEmpty</code> only checks for null values.

D <code>@NotNull</code> and <code>@NotEmpty</code> are interchangeable and have the same effect.

💡 Hint

Think about whether empty strings or empty collections are allowed.

@NotNull only checks that the value is not null but allows empty strings or empty collections. @NotEmpty checks that the value is not null and also not empty (length > 0 for strings, size > 0 for collections).

❓ component_behavior

intermediate

Validation behavior of @NotBlank on a string field

Given a Spring Boot model with a string field annotated with @NotBlank, what input value will cause validation to fail?

Spring Boot

public class User {
    @NotBlank
    private String username;

    // getters and setters
}

A "user123" (a normal non-empty string)

B " " (a string with only spaces)

C "null" (the string literal 'null')

D "user_1" (a string with underscore)

💡 Hint

Remember what @NotBlank checks for in strings.

@NotBlank fails validation if the string is null, empty, or contains only whitespace characters. A string with spaces only is considered blank.

📝 Syntax

advanced

Correct usage of @NotEmpty on a collection

Which code snippet correctly applies @NotEmpty to a list field in a Spring Boot model to ensure it is not null and not empty?

A

public class Order {
    @NotEmpty
    private List<String> items;
}

B

public class Order {
    @NotEmpty
    private String items;
}

C

public class Order {
    @NotEmpty
    private Map<String, String> items;
}

D

public class Order {
    @NotEmpty
    private int[] items;
}

💡 Hint

Consider which data types @NotEmpty supports.

@NotEmpty supports strings, collections, maps, and arrays. Option A uses a List which is a collection, so it is valid. Option A uses a string which is valid but the question asks for a list. Option A uses a Map which is valid but the question asks for a list. Option A uses an array, not a list.

🔧 Debug

advanced

Why does @NotBlank not work on an Integer field?

Consider this Spring Boot model:

public class Product {
    @NotBlank
    private Integer quantity;
}

Why will validation fail or not work as expected?

A <code>@NotBlank</code> only works on primitive types, not wrapper classes.

B <code>@NotBlank</code> works on all object types, so this should work fine.

C <code>@NotBlank</code> requires the field to be annotated with @NotNull as well.

D <code>@NotBlank</code> only works on CharSequence (strings), so it cannot validate Integer fields.

💡 Hint

Check the supported types for @NotBlank.

@NotBlank is designed to validate strings (CharSequence). It does not apply to numeric types like Integer. For numeric fields, @NotNull or other numeric validations should be used.

❓ state_output

expert

Validation result for a complex object with mixed annotations

Given this Spring Boot model:

public class UserProfile {
    @NotNull
    private String id;

    @NotBlank
    private String name;

    @NotEmpty
    private List roles;

    // getters and setters
}

Which input will pass all validations?

A {"id": "456", "name": "Bob", "roles": []}

B {"id": null, "name": "Alice", "roles": ["user"]}

C {"id": "789", "name": "Carol", "roles": ["editor"]}

D {"id": "123", "name": "", "roles": ["admin"]}

💡 Hint

Check each annotation's requirement for each field.

@NotNull requires id not to be null.
@NotBlank requires name not to be null, empty, or whitespace.
@NotEmpty requires roles not to be null or empty.
Option C meets all these conditions.